A great need exists today for wires of extremely small diameter or what are known as ultrafine wires. Such industries as the microelectronics industry require wires of a diameter of something substantially less than .001 of an inch. Such ultrafine wires are usually required to be of one of the precious metals such as gold, silver, platinum, etc.
One common method of producing fine wires is by the well known drawing technique. This technique has been highly refined and, in the case of the extremely fine wires, diamond dies have been employed. In this technique, a single strand of the wire material is successively reduced through dies. As the diameter continues to reduce, the technique becomes more tedious as the wire has less and less strength and drawing capability due to its extremely reduced diameter. As a practical matter, wires below .001 inches in diameter cannot be produced by this single strand-drawing technique. For a great number of applications, wires of this diameter are not small enough.
A further refinement in the single strand drawing technique has evolved in the state of the art. In this technique, a core wire of the particular metal from which the wire is to be formed is encased in a sacrificial sheath. The core and the sheath are then reduced as a unit to a predetermined small diameter. Thereafter, the sheath is removed from the wire by various sacrificial methods such as etching in an acid leaving the core wire.
In the core wire-sacrificial sheath technique, the ratio of the diameter of the core wire to the diameter of the sheath will remain constant through the successive reductions. Thus, for example, if the core wire is 1/10 the diameter of the sheath and the diameter of the sheath is reduced to 1/20 of its initial diameter, the core wire will likewise be reduced to 1/20 of its diameter. Accordingly, much greater reductions in the core wire are available by this technique since the overall sheath-core combination may be reduced by the conventional drawing technique to the same small diameters that the single strand of the core material itself could be reduced.
Theoretically, extremely large ratios of sheath diameter to core wire ratios could be utilized to produce core wires of diameter approaching microscopic dimensions. However, practical limitations exist in finding sheath material that will have an extremely large outside diameter and the requisite extremely small inside diameter of sufficient accuracy to permit practical use of the technique. Today sheath material which may be used in this technique can only be obtained with a ratio no greater than ten to one. This practical limitation on the sheath material results in a limitation on the smallest diameter to which the core wire can practically be drawn to a level of .001 of an inch. Attempts to draw the sheath-core combination to produce core wires of dimensions less than this becomes impractical or impossible.